Hyporheic flow, in which surface water and groundwater mix along fluvial corridors, is important for water quality and aquatic habitat. Longitudinal variation in the rates of exchange between surface and hyporheic water may be correlated with the width of the actively reworked floodplain. We estimate hyporheic flow rates along a 33-km reach of the upper Willamette River using dye tracers and discharge measurements. In May and June 1998, we released fluorescent dye above three contiguous river reaches 8-15 km long and measured the passage of the dye cloud at main channel, backwater, and groundwater locations. We based hyporheic flow estimates on transient storage analysis of the dye curves. We find more hyporheic flow in river reaches with both steep hydraulic gradients into the stream bed and high porosity exposed gravel areas. Locations with more hyporheic flow correspond to unconstrained channel reaches where main channel morphology is temporally dynamic. Channel reaches that are stabilized by natural and anthropogenic channel constraints have less hyporheic flow. Movement of the river channel across a wider active floodplain fosters increased hyporheic flow for improved aquatic ecosystem diversity.